Magnetostrictive transducer



1955 A. M DONALD ET AL MAGNETOSTRICTIVE TRANSDUCER 2 Sheets-Sheet 1Filed Oct. 19, 1953 Unite states std MAGNETGSTRICTIVE TRANSDUCER LeslieA. MacDonald, New Hyde Park, N. Y., and

Lawrence H. Kelly, Grange, N. J., assignors to American DistrictTelegraph Company, Jersey City, N. J., a corporation of New JerseyApplication October 19, 1953, Serial No. 386,862

12 Claims. (Cl. 31026) system wherein movement in a protected room orenclosure is detected. This system employs a transducer used as a loudspeaker or transmitter to emit vibrations, preferably above the audiblerange. These vibration waves reflect about an enclosed area and impingeupon another transducer arranged as a microphone or receiver. Because ofthe Doppler effect, any substantial movement within the area will causea variation in the frequency of the vibrational energy detected by thereceiving transducer whereby a signaling device may be actuated toindicate an intruder in the protected room.

The effectiveness of the above system depends in part upon effectivelyfilling the room with paths of vibrational energy. While conventionalmic ophones and loud speakers have been used with a certain amount ofsuccess, these devices have certain inherent drawbacks that limitmaximum efficiency and sensitivity of the system. For example, thediaphragm of a loud speaker has a relatively small area and sound wavesemanating therefrom are generally directed to take a definite pathrather than being permitted to radiate out in all directions as would bedesirable in a motion detecting system. Likewise, the conventionalmicrophone is similarly limited to receiving vibrations from definitedirections rather than from substantially all directions. A conventionalloud speaker in a motion detecting system will radiate vibration waveswhich will reflect about the protected area and eventually reach themicrophone. However, due to the directional limitations of suchtransducers, the number of reflections will be exceedingly large andhence the efficiency of the system is reduced. Further, withconventional transducers, the possibility is present that blind spotswill exist in the room where few vibrational energy paths will exist.The efiectiveness of such a system in detecting motion in such areaswould be minimized.

It is therefore a principal object of the present invention to providean improved vibration transducer particularly adapted for use in amotion detecting system.

More particularly, it is an important object of this invention toprovide a transducer of the magnetostriction type having highsensitivity and efliciency.

Another object of this invention is to provide an improvedmagnetostn'ction transducer wherein the magnetostriction effect ismaximized.

Other and further objects, features and advanta es of the presentinvention will become apparent from a reading of the followingdescription.

A transducer constructed in accordance with the invention comprises acoil, a rod of magnetostrictive material located within the coil, andresonating means mechanically coupled to the rod and having apredetermined natural frequency of vibration, the rod and the 'atentresonating means forming a vibrating system having a natural frequencyof vibration substantially equal to the predetermined frequency. Inaccordance with a further feature of the invention, the resonating meansmay be realized as two cylindrical elements each affixed adjacent themid point thereof to a respective end of the magnetostrictive rod. Themagnetostrictive rod of the invention may comprise a plurality oflaminae insulated from each other by means of thin strips of paper orthe like, the outer two laminae being channel-shaped with the flangesthereof extending outwardly to minimize transverse bending of the roddue to transverse magnetostrictive forces set up therein by thealternating current flowing in the coil.

The invention will now be described in greater detail with reference tothe appended drawings, in which:

Fig. l is a plan or top view of a transducer constructed in accordancewith the present invention and mounted on a vertical supporting member;

Fig. 2 is a front elevational view thereof with the cover plate removed;

Fig. 3 is a sectional view taken along the line 33 of Fig. 2;

Fig. 4 is a sectional view taken along the line 4-4 of Fig. 2; and

Fig. 5 is an end view of the coil spool showing the mounting screws forthe magnetostriction rod.

Referring now to the drawings, there is illustrated a preferredembodiment of a transducer constructed in accordance with the presentinvention. The principal elements of the transducer are a reflector Ill,a magnetostriction rod 11, a coil assembly generally designated by thereference numeral 12, and a pair of cylindrical resonating elements 13and 14.

The reflector comprises a central fiat portion 15 and curved endportions 16 and 17 adapted to reflect outwardly vibrational energyemanating from the resonators 13 and 14 or to direct toward theresonators received vibrational energy. The reflector 10 mayconveniently be affixed to a supporting member or wall 18 by screws 19.A generally rectangular housing 20 is afiixed to the central portion 15of the reflector 10 by means of screws 21. The screws 21 also serve toaflix a channel-shaped bracket member 22 to the rear of the housing 26.Brackets 23 and 24, which may be afi'lxed to diagonally opposite cornersof the housing 20 by any suitable means such as welding, are providedwith threaded holes 25 and 26, respectively, adapted to receive screws28 and 29, respectively, for affixing a cover plate 30 to the housing29. Side members 31 and 32 of the housing Zll are provided withelongated slots 33 adapted to accommodate the coil assembly 12.

The coil assembly 12 comprises a cylindrical spool or former 34 whichmay be made of any suitable insulating material such as, for example,polystyrene. Each end of the spool 34 is provided with flattenedopposite outer surfaces 35, as shown in Fig. 5, which are adapted to beengaged by the edges of the slots 33 to securely maintain the coilassembly 12 in place and to prevent the latter from turning. A bushing36, which may conveniently be made of nylon or the like, is locatedwithin the spool 34 at the center thereof to lend support to the coilassembly and to provide a secure mounting holes in the bushing 36. Thescrews 38 serve to position the magnetostriction rod 11 within the coilassembly 89 coincident with the axis thereof. For this purpose thescrews 38 contact the respective surfaces of the rod 11, preferably atthe mid point thereof.

The ends of the coil 37 are connected to respective terminals 39 and 40.The terminals 39 and 40 are fastened to a terminal block 41 by means ofscrews 42 and 43, respectively. The screws 42 and 43 also serve toconnect input leads 44 and 45 to the respective terminals 39 and 40. Ina transmitting transducer the input leads 44 and 45 will be connected toa source of alternating current having the desired frequency, whichmight be, for example, 22,000 cycles. In a receiving transducer, theleads 44 and 45 will serve as the output leads to supply to suitabledetecting or other equipment the alternating current generated in thecoil 37 by mechanical vibrations of the rod 11. The block 41 is affixedto the housing 20 by means of screws 46 and 47. The screw 46 mayconveniently be employed to afford a ground connection for the inputlead 44, as shown in Fig. 2.

The magnetostriction rod 11 comprises a plurality of lamina of nickel orother suitable magnetostriction material separated by sheets of waxpaper or other suitable insulating material. As is best shown in Fig. 4,the outer laminae 48 and 49 are channel-shaped in order to minimize themagnetostriction effect transversely of the rod. This construction ofthe outer laminae provides stiffness to minimize or eliminate transversebending of the rod 11 in operation. Magnetostriction rods or bars aregenerally held together by gluing or shellacking the laminae. Eithermethod produces a solid piece which must change dimensions as a unit. Insuch an arrangement energy may be wasted and longitudinal dimensionalchange may be limited by the requirement that energy must be expended inovercoming the longitudinal cohesive forces of the binding material. Inaccordance with the invention, the laminae are preferably not boundtogether by cohesive material but are separated by a smooth insulatingmaterial. As is best shown in Fig. 3, the upturned edges of thechanneled laminae 48 and 49 extend at each end to a point just short ofthe end of the rod 11. Cap members 50 and 51 are provided at each end ofthe rod 11 and may, if desired, be soldered or otherwise rigidly affixedthereto.

The cylindrical resonators 13 and 14, which may couveniently be made inthe form of thin-walled, wide diameter tubes, are each provided with alongitudinally centered hole adapted to receive rivets 52 and 53,respectively. The other ends of the rivets are mounted in the ends ofthe caps 50 and 51, respectively, so that the tubes 13 and 14 arerigidly secured to the caps 50 and 51, respectively, and hence torespective ends of the magnetostriction rod 11.

The transmitting transducer of the invention operates on the well-knownmagnetostriction principle that, when a ferromagnetic material is placedin a varying magnetic field, certain changes occur in its internalstructure and the resulting stresses on the specimen will cause smallchanges in its physical dimensions. The receiving transducer of theinvention operates on the converse of this principle that, when thematerial in the presence of a magnetic field is subjected to externalstress, its degree of magnetization is altered. The mechanical changescaused by variation in the induced magnetic field may be in the lineardimensions, the circular dimensions, or in the volume. The effect,however, is more pronounced in the linear dimensions of the specimen,the change of length being either in a positive (increasing) or negative(decreasing) direction, depending upon the magnetostrictive coefficientof the material used.

In the embodiment of the invention illustrated in the drawings, themagnetostriction rod 11 has been described as made up of thin strips ofnickel held together by cap members 50 and 51 and separated by thinstrips of an insulating material such as paper. When themagnetostriction rod 11 is made of nickel, as described, the negativemagnetostrictive coefficient of nickel will tend to cause the rod tocontract and return to its normal length twice during each cycle ofmagnetization. While somewhat less efiicient than nickel, Monel metal,which has a positive magnetostrictive coefficient, could be used for therod 11. Other magnetostrictive materials could be used, if desired.Since the magnetostrictive effect is independent of the direction of theapplied magnetic field, a permanent magnetic field may be employed toexert a steady polarizing effect on the rod 11. When the magnitude ofthis polarizing field is approximately equal to the amplitude of theinduced magnetic flux, both the mechanical and electrical frequencieswill be identical. Otherwise, the mechanical frequency would be doublethe electrical frequency. A permanent bar magnet 54 held against therear of the housing 20 by the channel member 22 may conveniently beemployed to secure the desired polarizing field.

The resonators 13 and 14, which may conveniently bemade of aluminum,should be dimensioned so that their fundamental frequency of vibrationwill coincide with the fundamental frequency of the alternating currentsupplied to the coil 37 through the conductors 44 and 45 or receivedfrom another transducer. Similarly, the length of the rod 11 should bechosen so that its fundamental frequency of vibration is approximatelyequal to that of the desired operating frequency, i. e., the frequencyof the current supplied to the coil 37 or the vibrational frequency ofreceived energy. The large vibrating surface offered by the resonators13 and 14 is very great compared to the cross-sectional area at each endof the rod 11, so that these resonators accordingly enhance theefficiency of the unit for transmission of sound waves in air.Similarly, when the transducer is used for receiving sound vibrations,the large vibrating surface area offered by the resonators 13 and 14will tend to maximize the fundamental frequency alternating currentvoltage induced in the coil 37 and supplied as an output to theconductors 44 and 45. The magnitude of the output voltage isproportional to the amplitude of vibration of the rod 11 and hence tothe amplitude of vibration of the resonators 13 and 14.

In general, it may be said that the rod 11 should be given a lengthapproximately equal to an integral number of half-wave lengths at theoperating frequency,

while the rod 11 should be supported at a nodal pointv thereof. Thenodal points will occur at odd quarterwave lengths from the ends of therod 11, and the nodal point selected for support is preferably at themid point of the rod. The half and quarter-wave lengths referred to arethose of the sound energy in air. In practice, the rod 11 may be madeslightly less than an integral number of half-wave lengths long so thatthe rod 11 together with the resonators 13 and 14 will form a systemhaving a natural frequency of vibration substantially equal to theoperating frequency. In addition to this system resonance, theresonators 13 and 14 should each have a natural frequency of vibrationsubstantially equal to the operating frequency. The shortening of therod 11 is desirable since, while the resonators 13 and 14 are verylight, their mass does have some effect on the natural period of thesystem. The theoretical length of the resonators 13 and 14, which willcause the same to be resonant at the operating frequency, may becalculated from the following formula:

1.133 TI K L E- Where ii is the fundamental frequency of operation, L isthe theoretical axial length of each resonator, E is Youngs modulus, Kis the radius of gyration, and P is the density of the material of whichthe resonator is formed. The radius of gyration K is equal to where a isthe outside radius of the cylinder and a1 is the inside radius thereof.The actual length required will be somewhat less than the theoreticallength because of the effect on natural frequency produced by theconnection to the end of the rod 11. It will generally be foundadvisable in each case to empirically determine the requisite shorteningfrom the theoretical length.

In a transmitting transducer considerable heat will be generated inoperation which Will to some extent vary the natural frequencies of therod 11 and resonators 13 and 14. The dimensions of these elements shouldtherefore be selected so that the natural frequencies under actualoperating conditions Will coincide with the frequency of the appliedalternating potential. In a receiving transducer these heating effectswill generally be negligible, so that the design thereof will not haveto take into account dimensional changes due to temperature rise.

When used as a receiving transducer in a motion detecting system,vibrations received by the transducer having a frequency slightlydifferent from that of the fundawith a specific embodiment thereof andin a specific use,

various modifications thereof will occur to those skilled in the artWithout departing from the spirit and scope of the invention as setforth in the appended claims.

What is claimed is:

1. In a motion detection system employing sound Waves having a givenfrequency, a transducer comprising a coil, a rod of magnetostrictivematerial located within said coil, and cylindrical resonating meansmechanically coupled to said rod and having a predetermined naturalfrequency of vibration substantially equal to said given frequency, saidrod and said resonating means forming a vibrating system having anatural frequency of vibration substantially equal to said givenfrequency.

2. A transducer, comprising a coil, a rod of magnetostrictive materiallocated within said coil, and cylindrical resonating means mechanicallycoupled to said rod and having a predetermined natural frequency ofvibration, said rod and said resonating means forming a vibrating systemhaving a natural frequency of vibration substantially equal to saidpredetermined frequency.

3. A transducer, comprising a coil, a rod of magnetostrictive materiallocated within said coil, and a pair of cylindrical elements eachaffixed to said rod at a respective end thereof and each having apredetermined natural frequency of vibration, said rod and saidresonating elements forming a vibrating system having a naturalfrequency of vibration substantially equal to said predeterminedfrequency.

4. A transducer, comprising a coil, a rod of magnetostrictive materialadapted to have longitudinal mechanical vibrations of given frequencyset up therein, means to support said rod within said coil at a nodalpoint of the rod with respect to vibrations of said given frequency, anda pair of cylindrical resonating elements each connected adjacent themid point thereof to a respective end of said rod whereby like frequencymechanical vibrations occur in said resonating elements and in said rod,said rod and said resonating elements forming a vibratory system havinga natural frequency of vibration substantially equal to said givenfrequency and said resonating elements each having a natural frequencyof vibration substantially equal to said given frequency.

5. A transducer, comprising a coil, a rod of magnetostrictive materialmagnetically coupled to said coil and adapted to have longitudinalmechanical vibrations of given frequency set up therein, means tosubject said rod to a polarizing magnetic field, means to support saidrod Within said coil at a nodal point of the rod with respect tovibrations of said given frequency, and a pair of cylindrical resonatingelements each connected adjacent the mid point thereof to a respectiveend of said rod whereby like frequency mechanical vibrations occur insaid resonating elements and in said rod, the axes of said resonatingelements being transverse to the axis of said rod, said rod and saidresonating elements forming a vibratory system having a naturalfrequency of vibration substantially equal to said given frequency andsaid resonating elements each having a natural frequency of vibrationsubstantially equal to said given frequency.

6. A transducer, comprising a coil adapted to have an alternatingcurrent passed therethrough, a rod of magnetostrictive material, meansto support said rod Within said coil whereby the varying magnetic fieldproduced by alternating current flow in said coil produces longitudinalmechanical vibrations of given frequency in said rod, and a pair ofcylindrical resonating elements each connected adjacent the mid pointthereof to a respective end of said rod whereby like frequencymechanical vibrations occur in said resonating elements and in said rod,the axes of said resonating elements being transverse to the axis ofsaid rod, said rod and said resonating elements forming a vibratorysystem having a natural frequency of vibration substantially equal tosaid given frequency and said resonating elements each having a naturalfrequency of vibration substantially equal to said given frequency.

7. A transducer, comprising a coil adapted to have an alternatingcurrent of given frequency passed therethrough, a rod ofmagnetostrictive material, means to subject said rod to a polarizingmagnetic field, means to support said rod Within said coil at a midpoint thereof whereby the varying magnetic field produced by thealternating current flow in said coil produces longitudinal mechanicalvibrations of said given frequency in said rod, and a pair ofcylindrical resonating elements each connected adjacent the mid pointthereof to a respective end of said rod whereby like frequencymechanical vibrations occur in said resonating elements and in said rod,the axes of said resonating elements being transverse to the axis ofsaid rod, said rod and said resonating elements forming a vibratorysystem having a natural frequency of vibration substantially equal tosaid given frequency and said resonating elements each having a naturalfrequency of vibration substantially equal to said given frequency.

8. A transducer, comprising a coil, a rod of magnetostrictive materialadapted to have longitudinal mechanical vibrations of given frequencyset up therein, said rod comprising a plurality of laminae insulatedfrom each other and having the two outer opposite laminae channel-shapedwith the flanges extending outwardly to minimize transverse bending ofthe rod due to transverse magnetostrictive forces set up therein, meansto support said rod Within said coil at a nodal point with respect tosaid given frequency, and a pair of cylindrical resonating elements eachconnected adjacent the mid point thereof to a respective end of said rodwhereby like frequency mechanical vibrations occur in said resonatingelements and in said rod.

9. A transducer, comprising a coil, a laminated rod of magnetostrictivematerial adapted to have longitudinal mechanical vibrations of givenfrequency set up therein, said rod having a length slightly less than anintegral number of half-Wave lengths at said given frequency, means tosupport said rod within said coil at a nodal point of said rod withrespect to vibrations of said given frequency, and a pair of cylindricalresonating elements each connected to a respective end of said rodwhereby like frequency mechanical vibrations occur in said resonatingelements and in said rod, said resonating elements each having a naturalfrequency of vibration-substantially equal to said given frequency.

10. In a vibration transducer having a coil, a magnetostriction rodlocated within the coil and comprising a plurality of laminae insulatedfrom each other, the outer opposite two laminae being channel-shapedwith the flanges thereof extending outwardly to minimize transversebending of the rod due to transverse magnetostrictive forces set uptherein by alternating current flowing in said coil.

11. In a vibration transducer having a coil, a magnetostriction rodlocated within the coil and comprising a plurality of laminae, and aplurality of thin, smooth insulating members separating adjacentlaminae, the outer opposite two laminae being channel-shaped with theflanges thereof extending outwardly to minimize transverse bending ofthe rod due to transverse magnetostrictiveforces set up therein byalternating current flowing in said coil.

12. A transducer, comprising a coil, a laminated rod of magnetostrictivematerial adapted to have longitudinal mechanical vibrations of givenfrequency set up therein, said rod having a length slightly less than anintegral number of half-wave lengths at said given frequency, means tosupport said rod within said coil at a nodal point of said rod withrespect to vibrations of said given frequency, a pair of cylindricalresonating elements, means to connect each of said resonating elementsto a respective end of said rod whereby like frequency mechanicalvibrations occur in said resonating elements and-in said rod, vsaidresonating elements each having a natural frequency of vibrationsubstantially equal to said given frequency and said securing meansproviding a substantial point contact between the resonating elementsand the respective ends of said rod, and a curved reflector spaced fromsaid resonating elements to direct vibrational waves.

References Cited in the file of this patent UNITED STATES PATENTS1,689,121 Ferdon Oct. 23, 1928 2,519,345 Blanchard Aug. 22, 19502,539,535 Espenschied Jan. 30, 1951 2,655,645 Bagno Oct. 13, 1953

